1. Field of the Invention
The present invention relates to an ultrasonic flow meter that measures the flow volume of a liquid flowing through a pipe using ultrasonic waves.
2. Description of the Related Art
An ultrasonic flow meter is known in the prior art that uses ultrasonic waves to function as a flow meter that measures the flow volume of a liquid flowing through a pipe.
This ultrasonic flow meter provides two measuring units having a transducer at an interval in the lengthwise direction on a measuring pipe through which liquid flows. Ultrasonic waves are emitted from one of the transducers which are then received by the other transducer. Alternatively, ultrasonic waves are emitted from the other transducer and then received by the first transducer. The flow rate of the liquid in the measuring pipe is determined from the difference in propagation times of these ultrasonic waves, and flow volume is then measured from this flow rate.
However, if this ultrasonic flow meter is subjected to vibrations from the outside between the respective measuring units, measurement error occurring resulting in fluctuations in characteristics of the measurement data and causing problems that the flow volume cannot be measured accurately.
In addition, since the acoustic velocity, namely the velocity of the ultrasonic waves, changes according to the temperature of the liquid, it is necessary to measure flow volume using a conversion value corresponding to the temperature of the liquid. However, if ultrasonic waves emitted from the transducer are influenced by factors other than the temperature of the liquid, such as the outside ambient temperature, although the flow volume was corrected by converting according to the temperature of the liquid, there is the problem that the acoustic velocity is changed due to slight changes in temperature and the flow volume cannot be measured accurately.
The following provides a detailed explanation of changes in the acoustic velocity caused by changes in the temperature of the liquid using the drawings.
FIG. 9 is a graph showing the relationship between the temperature (xc2x0 C.) of water (liquid) and the acoustic velocity (m/s). In addition, FIG. 10A is a graph showing the change in a reference flow volume for each passage of time T in the case of a water temperature of 20xc2x0 C., while FIG. 10B is a graph showing the output of the transducers relative to the reference flow volume of FIG. 10A. In addition, FIG. 11A is a graph showing the change in a reference flow volume for each passage of time T in the case of a water temperature of 29xc2x0 C., while FIG. 11B is a graph showing the output of the transducers relative to the reference flow volume of FIG. 11A.
Furthermore, the units of flow volume Q and the reference flow volume shown in FIGS. 10A and 11A indicate flow volume per minute (mL/min), and the reference flow volume indicates the flow volume flowing through the measuring pipe of the ultrasonic flow meter obtained with a calibrated flow meter.
Conversion values of flow volume relative to the output of the transducers are obtained from the graphs shown in FIGS. 9 through 11B.
It is generally known that the acoustic velocity of ultrasonic waves output from the transducers changes considerably according to the temperature of the liquid, and can be represented in the graph showing the relationship between temperature and the acoustic velocity of FIG. 9. According to the graph shown in FIG. 9, the acoustic velocity can be seen to increase the higher the temperature of the liquid.
In consideration of this change in the acoustic velocity due to temperature, as shown in the graph of FIG. 10A, water at a temperature of 20xc2x0 C. is allowed to flow in two stages of 1000 mL and 500 mL per minute from time 0 through the measuring pipe of the ultrasonic flow meter using the reference flow meter. For the flow volume of the former first stage, the water is allowed to flow for time interval T1, and for the flow volume of the latter second stage, water is allowed to flow for time interval T2 so as to be continued from the first stage.
Whereupon, as shown in FIG. 10B, although the output of ultrasonic waves outputted from the transducers at the ambient temperature of 24xc2x0 C. remained nearly level prior to time 0 before the water flows (see A), it can be seen to decrease suddenly by displacement D1 (see B) corresponding to the start of water flow (time 0). When the flow volume of the water changes from 1000 mL/min to 500 mL/min (see C), the output can be seen to only change slightly by displacement D2.
As shown in the drawings, the difference in the output between displacement D1 and displacement D2 is such that D1xe2x96xa1xe2x96xa1D2, and the change in the output due to the temperature change of the difference of 4xc2x0 C. between the ambient temperature and the water temperature can be understood to be larger than the change in the output during the change in flow volume.
Next, an explanation is provided of the graphs in the case of allowing water at a water temperature of 29xc2x0 C. to flow as shown in FIG. 11 in comparison with the graph of FIG. 10. As shown in FIG. 11A, water at a temperature of 29xc2x0 C. is allowed to flow in two stages at 1000 mL/min and 500 mL/min starting at time 0 through the measuring pipe of the ultrasonic flow meter using the reference flow meter. For the flow volume of the former first stage, the water is allowed to flow for time interval T3, and for the flow volume of the latter second stage, the water is allowed to flow for time interval T4 so as to be continued from the first stage.
Whereupon, as shown in the graph of FIG. 11B, although the output of ultrasonic waves output from the transducers at an ambient temperature of 24xc2x0 C. was at the same position and remained nearly level (see E) at the stage of time 0 before the water flowed in the same manner as FIG. 10A, it can be seen increase suddenly by displacement D3 (see F) corresponding to the start of the flow of water (time 0). When the flow volume of water changes from 1000 mL/min to 500 mL/min (see G), the output can be seen to only change slightly by displacement D4.
As indicated in the drawings, the difference in output between displacement D3 and displacement D4 is such that D3xe2x96xa1xe2x96xa1D4, and the change in the output caused by a temperature change of the difference of 5xc2x0 C. between the ambient temperature and water temperature can be seen to be larger than the change in the output for the change in flow volume.
In this manner, in the ultrasonic flow meter, changes in flow volume are captured in an output region that is much smaller than the change in the output of the transducers resulting from a change in the liquid temperature. It can also be understood that the greater the difference between ambient temperature and liquid temperature, the larger the change in the output of the transducers.
Thus, if the liquid temperature is influenced even minimally by the external ambient temperature, the output of the transducer changes considerably, and measurement of flow volume at an extremely small displacement for this output of the transducers has a high potential to invite measurement error.
In this manner, in the conventional ultrasonic flow meter, there were cases in which it was difficult to accurately measure flow volume depending on the ambient temperature.
In consideration of the above circumstances, the object of the present invention is to provide an ultrasonic flow meter that is able to minimize effects caused by external vibrations, and accurately measure flow volume without being affected by outside temperature.
In order to achieve the above object, the present invention provides an ultrasonic flow meter comprising: a measuring pipe through which a liquid flows, and two measuring units provided on the measuring pipe at an interval in its lengthwise direction and which measures flow volume by determining the flow rate of the liquid from the difference in propagation times of ultrasonic waves in both directions between these measuring units; wherein, the measuring pipe is supported on a support stand, on which a pair of mounting units are provided on a base at a wider interval than the measuring units, by retaining the measuring pipe in the mounting units provided at an interval to the outside of the measuring units in an axial direction of the measuring pipe.
In this manner, since the measuring pipe is retained and supported at the outside of the measuring units in the axial direction of measuring pipe by mounting units provided on the base, external vibrations can be blocked with the mounting units, thereby allowing the reliability of measurement between the measuring units, which are susceptible to the effects of external vibrations, to be enhanced.
In the ultrasonic flow meter of the present invention, it is preferable that the mounting units have a first mounting member and a second mounting member that are fixed to be mutually facing, and retaining indentations in the shape of a circular arc, which form an opening that holds the outer periphery of the measuring pipe when mutually facing, are formed in these first and second mounting members.
In this manner, by mutually facing the first and second mounting members in the state in which the measuring pipe is arranged in retaining indentations formed in the first and second mounting members, the outer periphery of the measuring pipe can be retained extremely easily.
In the ultrasonic flow meter of the present invention, it is preferable that the mounting members have a lower mounting member and an upper mounting member fixed to be mutually facing above and below, and retaining indentations in the shape of a circular arc, which form an opening that holds the outer periphery of the measuring pipe when mutually facing, are formed in these lower and upper mounting members.
In this manner, by mutually facing the upper and lower mounting members in the state in which the measuring pipe is arranged in the retaining indentations formed on the upper and lower mounting members, the outer periphery of the measuring pipe can be retained extremely easily.
In the ultrasonic flow meter of the present invention, it is preferable that the inner diameter of an diameter of an opening which is composed by the retaining indentations is slightly smaller than the outer diameter of the measuring pipe.
In this manner, since the opening comprised by the retaining indentations formed by mutually facing each mounting member is formed to have a diameter that is slightly smaller than the outer diameter of the measuring pipe, the measuring pipe can be reliably retained by both mounting members.
In the ultrasonic flow meter of the present invention, it is preferable that the retaining indentations are formed to have a rugged surface.
In this manner, since the retaining indentations are formed to have a rugged surface, the measuring pipe can be reliably retained, and the effects of external vibrations can be further reduced.
In the ultrasonic flow meter of the present invention, it is preferable that the surface of the retaining indentations has a rugged shape as a result of forming engaging grooves along the peripheral direction.
In this manner, since the surface of the retaining indentations is formed to have a rugged shape as a result of forming engaging grooves along the peripheral direction, vibrations in the measuring pipe in the axial direction can be reliably blocked by the engaging grooves.
In the ultrasonic flow meter of the present invention, it is preferable that the engaging grooves are V-shaped grooves.
In this manner, since the engaging grooves are V-shaped grooves, the outer peripheral surface of the measuring pipe reliably engages with the engaging grooves, thereby reliably retaining the measuring pipe in the mounting members.
In the ultrasonic flow meter of the present invention, it is further preferable that a plurality of the engaging grooves are formed in the retaining indentations at intervals in the axial direction of the measuring pipe that is retained.
In this manner, since a plurality of engaging grooves are provided arranged in the axial direction, vibrations transmitted to the measuring pipe can be blocked even more reliably.
In the ultrasonic flow meter of the present invention, it is preferable that an insulating means that covers the measuring units and suppresses the transfer of heat from the outside is provided.
In this manner, since the measuring units are covered by the insulating means, it is difficult for external heat to transfer to the measuring units, and the temperature of the measuring units is maintained. In other words, the temperature of the liquid flowing through the measuring pipe constitutes the main effect on the measuring units. Therefore, the temperature of the liquid flowing through the measuring units is no longer affected by the external ambient temperature, and flow volume can be measured accurately from changes in the acoustic velocity of ultrasonic waves in accordance with the liquid temperature.
In the ultrasonic flow meter of the present invention, it is further preferable that the insulating means is composed of an insulating material.
In this manner, since the insulating means is composed of the insulating material, heat insulation is carried out more effectively, and for example, the measuring units can be covered according to the shape of the measuring units to promote more effective heat insulation. Therefore, it is possible to accurately measure flow volume without the liquid temperature being affected by the external temperature.
In the ultrasonic flow meter of the present invention, it is further preferable that the insulating means is a case that houses the measuring units.
In this manner, the measuring units are removed from transfer of heat from the outside by being housed, and therefore, flow volume can be measured accurately by blocking the effects of the external ambient temperature. In addition, the measuring units can be protected by the case, and the reliability and durability of the ultrasonic flow meter are improved. The case preferably uses a material having insulating action and a low coefficient of heat transfer, and preferably uses a material such as SUS.